I would not answer. If it's court ordered he call their phone, just don't answer. Tell him, ONE TIME, that the parenting plan states he call their phone, not yours.
My kids have learned that if they feel like answering or texting their dad back, they do. If not, they don't. It drives him nuts that they won't be at his beck and call, but now that he is usto it, he does not keep throwing a fit. THEY explained that they are busy with school, and a life, and they are not glued to their phones. I was proud of them. They also learned how to put their phones on do not disturb so it won't keep alerting them with his text if they don't feel like engaging with him. So far, so good.
Just because the phone is ringing does not mean you have to pick it up, nor do your children.
Hugs. Be strong. Once he sees that he is not getting under your skin anymore, he will stop being such a pest, unfortunately, he will likely try something else. Stay calm, and don't let him disrupt your life.
And...if you are worried, have the kids set a specific time with him, say two times a week, at a certain time, to talk, on their phones. That way, they know when it's coming, and so will he. He won't be able to accuse you of blocking the kids.